This happens from time to time when you don't properly set your emails encoding. Usually, you'll want to use UTF-8 or ISO-8859 and unless you set this specifically before sending it, your clients inbox will use whatever encoding they have set as default. This is probably why he's the only one receiving those other characters.

Sometimes you see this on webpages when you see oddly placed ? marks instead of quotes. Same thing. Just make sure to set the encoding and you should be good